- Week 3, Immunity System

One of the main components of our body, is the immunity system. Responsible for identifying and acting against diseases, the immunity system acts by detecting a "wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own healthy cells and tissues in order to function properly" (Wikipedia).

Before knowing how the system works, it is crucial for us to understand the causes for such functioning. Our immunity acts against pathogens, which are any living organisms or viruses capable of causing diseases. Some examples of pathogens are bacteria, viruses, fungi, protists and worms. (Photo: bacteria E Coli)

Taking into account bacteria, we can also start comparing the use of antibiotics when treating bacteria itself, and viruses. While antibiotics are effective against bacteria, they are not against viruses. This is due to their structures and normal functions. For example, bacteria are prokaryotic cells, therefore, they have a metabolism and a cell wall. Antibiotics act by blocking such metabolic pathways found in bacteria. Viruses on the other hand, are not cells, given that they are made of only a capsid and nucleic acid. Also, seeing they don't have it, viruses reproduce using the host cell's metabolic pathways, which are not affected by antibiotics, given that they are from our own body.

So how does the immunity system work?

To start up with, the immunity system is something that takes place even if we do not realize it is actually helping us. Starting with our first lines of defense, the process of immunity and avoiding pathogens within our body already takes place in our skin. Our skin is made of two layers; epidermis and dermis, the first one being impermeable, having a lower pH and already having other bacteria in it, which prevents other ones from growing. If such mechanism still fails, chemical secretions such as mucus, trap and kill microbes.

This second line of defense contains anti-bacterial organelles such as lysosomes are also present in mucous membranes, which further helps the body defense against pathogens. Phagocytic leucocytes, also known as macrophages, are present in the mucus, and help by ingesting large white blood cells that engulf pathogens. Finally, mucus, also helps when cilia sweep it back to the throat, leading germs to go to stomach to get destroyed by the acid environment.

Click here to check video.

Additionally, the immune system is also composed of antibodies and antigens. Antigens are substances, macromolecules that provoke an immune response through lymphocytes. On the other hand, antibodies are proteins secreted by plasma cells that bind to an antigen, recognize it and marks it for elimination. Antibodies are made by lymphocytes, which recognize an enormous number of antigens, but each individual cell recognizes only one type of antigen. Each lymphocyte puts some of the antibody that it makes into its cell surface with the antigen-combining site projecting outwards. Antibody production starts when a pathogen enters the body, its antigens bind to the antibodies in the cell surface of one type of lymphocyte (macrophages engulf it). Then the antigen is incorporated to macrophage membrane, which will be presented to helper T-cells. These helper T-cells then bind to the antigen located on the macrophage, and immediately activate T-cells, which will activate B-cells. Seeing that antibodies are produced in response to specific antigens, they will be made by B or plasma cells; they will be B-cells clones. Such clones will later spread into plasma cells and memory cells, the first one producing specific antibodies to the specific antigen. Some of the cells develop into short-lived plasma cells that secrete antibody specific for the antigen. Others develop into long-lived memory cells that can respond rapidly upon subsequent exposure to the same antigen.

Finally, the issues that IB most relates to the immune system is AIDS, caused by the HIV virus. HIV became really popular due to its easy transmission. It involves the transfer of body fluids, such as blood, placenta, or milk from breast feading, from an infected person to an uninfected one. HIV can also be sexually transmitted, or from the use of contaminated needles, or blood transfusions. Additionally, the problem of AIDS is not merely the virus itself, but also the implications involved in such. Individuals infected with HIV may not find partners, housing or employment. Also, reproduction activity in a population may be reduced because of the fear of AIDS, and families may become poorer if the individual with AIDS was the wage earner and is refused life insurance. As I have already watched it, I would highly recommend everyone to watch the movie Philadelphia, click here to see the trailer, given that it gives a fair and new insight in the life of a person who is affected by the virus and how it has changed his life.

- Week 2, the Transport System

The Transport System, which is also known as Circulatory system, is made up of vessels and muscles that help and control the flow of blood around the body. The transport system is mainly consisted of blood, blood vessels and the heart, which is responsible for the pumping and spreading of blood throughout the body. Although it is only one transport system, there are two types of circulation; pulmonary and systemic.

Systemic circulation is a process where oxygenated blood is carried away from the heart, towards all the parts of the body and returns with deoxygenated blood back to the heart. As the blood begins to flow, it leaves the heart through the left ventricle and goes through the aorta.

The artery aorta is the largest artery in the body, therefore, the pressure inside it is greater than the pressure within other arteries. Also, when leaving the heart, the aorta gets segmented into three branches, allowing blood to reach capillaries from all over the body. When blood finally reaches its destination, delivering oxygen to

all the cells, it comes back to the heart through the vena cava and it is, obviously, deoxygenated.

The pulmonary circulation is the one following the order where blood goes through the heart, followed by lungs and back to the heart again. In other words, as its name mentions, "pulmonary" (from Latin, pulmonarius) refers to lungs, therefore, it is the process involving such organs. As the systemic circulation ends, the pulmonary one starts by leaving the heart with deoxygenated blood through the pulmonary artery and towards the lungs, where an exchange will happen between oxygen and carbon dioxide.

An essential element of the circulatory system is the presence of blood.

Blood itself is composed essentially of three parts; plasma, which is the fluid; red blood cells that transport O2 and CO2; white blood cells belonging to the immune system; and platelets, cell fragments that help blood clotting.

Additionally, the transport system is also highly dependent on the heart. Having four different chambers, the heart is mostly divided into left and right; left side being the one responsible for the systemic transport, and the right one for the pulmonary circulation. As seen in the right diagram, the heart has many vessels, those of which receive a different name.

However, although they serve different purposes, they can also be generally ca

tegorized into three main segments; arteries, veins and capillaries. The first one, arteries, carry blood away from the heart through their strong thick walls, smooth muscle and small lumen, which increase the pressure. Veins on the other hand, carry blood to the heart, thing of which I personally relate to the portuguese word "vem" (comes), through their thin wall and large lumen. Also, given that the blood is coming to the heart, pressure is lost during the process (it loses through the segmentations of arteries/veins, distance traveled...), resulting in a smaller pressure within the vein itself. Finally, capillaries are what connect arteries and veins. They also have a wall of one-cell thickness, which allows a faster diffusion of materials across the membrane.

Finally, in order for the blood to reach every cell in the body, it has to be pumped by the heart with enough pressure and force that will allow its travel. In other words, the heart must beat. The pacemaker is located in the wall of the right atrium at the Sinoatrial node (SAN). Each time the pacemaker sends out a signal, the heart carries out a contraction or a beat. The heart beats by itself (myogenic). Nerves and hormones can transmit messages to the pacemaker: Sympathetic nerve secretes adrenaline (also known as "epinephrine") which carries messages from the brain to the pacemaker telling the pacemaker to speed up the beating of the heart. Another nerve tells it to slow it down, and Adrenalin, carried to the pacemaker by the bloodstream tells it to speed up the beating of the heart.

- Week 1, Digestion

This week in Biology class we learned about the process of digestion and all the "aspects" involved in such. To digest something is to break down nutrients such as proteins, carbohydrates and lipids. The process of digestion is a series of chemical reactions where ingested food is broken down into smaller and smaller forms. For example, proteins, carbohydrates and lipids are broken down into amino acids, monosaccharides and fatty acids, respectively, with the help of enzymes.
When enzymes are involved in the digestion process of nutrients, it is called a chemical digestion. On the other hand, nutrients such as vitamins, minerals and water don't need to be digested, once they are absorbed by the human body.

The digestion process starts with ingestion, which takes place in the mouth. When food is eaten, teeth break it into smaller molecules, process also known as mechanical digestion. Mechanical digestion may also occur inside the mouth due to the enzyme salivary amylase (present on the saliva produced by the salivary glands), which can break down starch. Nevertheless, after this first breakage of food, it moves to the esophagus and forward with peristalsis; involuntary contractions that push food down. Peristalsis can also be considered a part of mechanical digestion, once it squishes and breaks food.

Passing through the muscular tube which is the esophagus, food goes to the stomach, where both mechanical and chemical digestions will occur. Such digestions are reflected on the mashing and mixing of food and in the pepsin digestion of proteins. Additionally, the stomach is filled with HCl, a gastric acid which helps the breakage of nutrients, activates the protease pepsin and acts as a barrier against germs. In order to avoid the leaking of such acid, the walls of the stomach are filled with mucus. After food passes through the stomach, it goes to the liver, where the gallbladder is responsible for the storage of bile, which separates fat into smaller parts, hence, allowing its future absorption. Followed by the liver, there is the pancreas, the most active site for chemical digestion. The pancreas produces all types of enzymes, such as amylase, lipase, trypsin, and dipeptidase, and it also produces insulin, that controls the amount of sugar in the blood. The rich juices from both the pancreas and the gallbladder are later received by the deodenum located in the small intestine. The small intestine has villi, finger-like extensions, which greatly increase the surface area for absorption of molecules such as glucose, amino acids and fatty acids. When such nutrients are absorbed, they are distributed to the cells through the blood, also due to villi. Villi are made of lacteal, small vessels of lymphatic system, and capillary beds, small vessels of circulatory system. In this way, nutrients that are absorbed easily go to the blood, circulatory system, and are directed to a wide variety of body cells.

Finally, whatever was not absorbed by the small intestine is taken to the large intestine. As mentioned before, water was not digested, however, it is only absorbed at the end of the process, in the large intestine. This second intestine also allows bacteria to produce vitamins B and K, and acts in pre

paration for undigested, or unabsorbed, food to be eliminated. In the large intestine such waste material is compacted and is sent to the rectum, which will later release it through the anus.

A nice and explanatory video on digestion can be seen if clicked here.